When a sound wave strikes the spheres at one end of the acoustic lens, the sound gets converted into a type of shock wave known as a solitary wave.

The solitary wave propagates through the chains in the same way motion moves through the balls in a Newton's cradle. But because of the length of the chains, the solitary wave exits the last sphere as a sound wave instead of bouncing back through the chain.

Converting the sound wave into a solitary wave is crucial, because solitary waves are easier to control, said Daraio, a physicist at the California Institute of Technology. (Related: "Laser 'Light Bullets' Made to Curve.")

For example, by squeezing the balls in a chain closer together—a process called static precompression—scientists can adjust the solitary wave so that the emitted sound wave travels in a particular direction and at a given speed.

"The goal was to create an acoustic lens that could achieve very high focal intensities and at the same time be able to control the focal position without having to change the structure of the lens itself," Daraio said.

Rapid-Fire Sound Bullets

By tweaking each chain of metal balls in the acoustic lens separately, a barrage of sound bullets can be made to converge onto a single spot.

"We squeeze the outer chains of the lens more than the chains in the middle, and this causes the solitary waves to travel faster in the outer chains than in the inner ones," ultimately releasing successive sound bullets, Daraio explained.

In addition, changing certain parameters in the machine allows scientists to alter the intensity of the sound bullets: The emitted waves can be gentle enough to probe internal organs or powerful enough to serve as "sonic scalpels" for cleaving off tumors.

As an imaging tool, the acoustic lens beats ultrasound imaging because the sound pulses can be focused much more tightly and can be easily repositioned.